IMHO, bottom boards are the one piece of a hive hat you don't really need to use piano-building skills on. They get beat up over time no matter how careful you are, cleaning, scraping, moving, etc.
Working on 4 years on some plywood bottoms. 2 coats of exterior grade paint.
Perry is right about them getting beat up over time, but that applies to any bottom board. Plywood or tongue and groove, or boards glued etc..
I get ten bottom boards out of one sheet of plywood. It costs me $2.50 to make a bottom board AND paint it, labor not withstanding...
I stopped trying to be fancy and pretty, and went with functional and replaceable. If I can get 3 or 4 years out of a $2.50 bottom board it earned its place. I have commercial bottom boards with rabbited sides, tongue and groove bottoms going into their third year that are going to need replaced this coming year... So, a lot depends on what you want, and how you take care of your woodenware. I often see something that needs replaced during an inspection, but dont have a spare with me... one thing leads to another, and the damage reaches the point I HAVE to do something.. Typically.... plywood neglected WILL go to heck faster than 3/4 inch boards neglected.
Making new out of boards doesnt cost much more than the plywood.
The difference then?
It is in the making... It takes me a few minutes to slap the plywood bottom board together, and several minutes to put one together with boards.. the LazyBkpr at work!